In optical fibre systems there are at different points required optical connections between the optical elements included in the system. Thus, there is required connections between different lengths of optical fibres, between an optical fibre and a light source, for example a diode, between an optical fibre and a light detector and so on. Each connection must be established while using very great accuracy if it shall be possible to keep the losses at a minimum. Of course, the problem of providing a correct connection is increased by the fact that optical fibres have a very small cross sectional area. Thus, the small cross sectional area provides that substantial losses appear also at small deviations from a correct relative position between the two fibre end portions, i.e. at deviations from the position in which the fibre end portions are positioned on a common optical axis. Also dirt represents a substantial risk as the dirt can easily reduce the already small transmission area.
Different methods for providing a correct connection of optical fibres with each other and with other optical elements have been suggested. In such previously known methods there are used lenses in which the light beam is magnified so that the light transmission in the connection takes place at a sectional area which is magnified several times providing for a corresponding reduction of the sensitivity to deviations from a correct position, dirt and so on. Thereby, it is a drawback that the connection of for example two fibre end portions to each other requires instead of one single connection, i.e. the connection between the fibre end portions, an optical connection of the fibre end portions to one lens each and an optical connection of the lenses to each other, i.e. three connections. Thereby, it is important that the optical fibres and the lens are positioned on the same optical axis as well as that the end surfaces of the fibres are positioned in the focus of the lens in question. Thus, the connection between the optical fibres and the lenses constitutes the most sensitive part of the connection system while the optical connection of the lenses to each other can easily be provided without errors.
Especially in such connections in which there is spaces between fibre/fibre, fibre/lens and lens/lens a fraction of the light is reflected back through the optical elements towards the light source. In some cases this results only in a loss in the direction of transmission of the light corresponding to the light which is reflected backwardly. In other cases in which for example a laser diode is used as a light source the light reflected back towards the laser diode is fatal to the transmission. The reflection disturbs the laser so that this is prevented from sending digital impulse sequences in a way free from interferences.
In 1-fibre systems in which also a fibre from a photo diode is connected with the transmission fibre reflections can disturb a receiving function which is intended for receiving external signals.
There are different previously known methods for reducing the reflections, i.e. for increasing the return damping to a value of about 30 dB or more, and if the effect of the reflections is decreased to such low values they are too week to be able to disturb the transmission of light impulses from the laser diode.
Low reflection levels can be obtained by providing an optimum value of the light transmission in the transmission direction of the light source. Such an optimum value can be provided by anti-reflection treatment of all surfaces extending transversely of the optical axis including such surfaces of the fibres which extend transversely. The anti-reflection treatment can for example be provided by coating the surfaces with magnesium fluoride. However, the anti-reflection treatment of the fibre end surfaces is a very impractical method with regard to the fact that the user of an easily applicable fibre connector device is forced to complicated operations before the fibre can be mounted in the connector device.
It is also possible to provide index matching means, such as gel, transparent silicon rubber pads and the like between the optical elements with the object of eliminating the air space between the elements and thereby reducing the reflections. However, it is a drawback that these agents at the same time reduce the transmission level in the direction of transmission as these agents per se constitute new optical elements having worse light transmission performances than optical glass, fibre materials and the like. In impure condition they can also cause a heavily decreased transmission by introducing dirt particles into the optical axis. Also aging of such index matching agents can provide the agent with bad optical properties.
It is possible also to reduce reflections in connectors between lens and fibre by positioning the elements in engagement with each other and thereby eliminating the air space. In this method there is used such a glass in the lens that the focus will be positioned in the surface of the lens. However, this provides only for a partial reduction of the reflections and not a reduction to the low levels as required.